High-frequency oscillator



Sept 4, 1951 P. o. FARNHAM 2,566,606

HIGH-'FREQUENCY OSQILLATOR I n Filed Feb. 11, 1947 2 Sheets-Sheet 1 2 ShISBiS-Sheet 2 Sept 4, 1951 P. o. FARNHAM I HIQH-FREQUENCY OSCILLTOR Filed Feb. 11, 1947 'corresponds f peller for any given klystron mode (1%, 2%, 3%, etc.)

Patented Sept. 4, 1951 HIGH-FREQUENCY OSCILLATOR Paul O Farnham, Mountain Lakes, N. J., assigner l to Aircraft Radio Corporation, Bocnton, N. J., a corporation of New Jersey Application February 11, 1947, Serial No. 727,885

(Cl. Z50-36) 6 Claims.

This invention relates to velocity modulated vacuum tubes for ultrahigh frequency applications and more particularly to the type known generally as a reux klystron.

The reflex klystron, which is widely used in the microwave field such as for example in radar apparatus as a tunable oscillation generator in the radar receiver, includes an electron gun for producing a beam ofr electrons, a tunable resonant cavity having a pair of spaced resonator grids for forming a radio frequency nel-d, a drift space and a repeller electrode. In operation, the electron beam upon leaving the gun passes into the radio frequency eld formed between the resonator grids and the electrons are velocity modulated by this eld to form electron bunches. The R. E. field set up by the resonant cavity is commonly referred to as the buncher. After the buncher, the electrons pass into the drift space and are then reversed in direction by the repeller electrode. The reflected electrons, now converted into a density modulated beam by the retarding effect of the eld set up by the repeller relectrode whose potential is negative with respect to the cathode element of the electron gun,

pass back through the buncher which now acts as a catcher and the density modulated beam is then used to deliver ultrahigh frequency oscillation energy to the resonant cavity and to any useful load circuit coupled to it.

The reflex klystron is tuned for operation at different frequencies by varying the dimensions ofthe resonant cavity and by varying the repeller potential so that the time required for a mean velocity electron to travel out into the drift space and back into the resonator grid space closely to a number of radio frequency cycles equal to 1%, 2%, 3%, etc. This number will be termed the mode number. As the size of the cavity is changed, its resonant frequency characteristic is also changed to thereby change the frequency of the R. F. field which in turn effects a corresponding change in the velocity modulation in the electron beam.

It has been found that the direct current revoltage required on the repeller electrode will Vary almost linearly with the oscillation frequency' of the klystron, the latter being mechanically controlled by the adjustment of a plunger which forms one wall of the resonant cavity. Since the wave length of the oscillations produced in the klystron varies as almost a linear function of the setting of the plunger, and the wave length varies inversely as frequency, it has been found necessary for a single control tuning, i. e. tuning in which the repeller electrode voltage is varied simultaneously with changes in setting of the plunger elementof the resonant cavity by a single control element, to introduce a non-linear reciprocal law transformation between the motion of the plunger and the value of the repeller voltage. In the past, such trans'- formation has been accomplished by different methods, all of which have in common some form of voltage divider for obtaining the. required variable repeller voltage, the control arm of the voltage divider being mechanically coupled with the device for changing the setting of the plunger so that both are varied simultaneously by a common drive.

According to one arrangement, the voltage divider was a potentiometer constructed in such manner that the resistance between one end of the potentiometer and the contact point of the potentiometer arm was a non-linear andreciprocal function of the displacement of the contact arm, and a direct linear mechanical drive was used to couple the plunger motion with the motion of the contact arm.

According to another' arrangement, the potentiometer element was of the so-called linear resistance type i. e. its resistance varied linearly with displacement of the contact arm, and the mechanical linkage between the drive for the plunger element for the resonant cavity and the contact arm was designed to produce the required reciprocal transformation by means vof non-constant radius meshing gears, or by specially shaped cams, or by special linkage movements.

Another arrangement has been to use a linear type of potentiometer with a linear drive between the plunger adjusting deviceand the potentiometer contact arm but with taps ldistributed along the winding of the potentiometer connected individually through properly valued separate series fixed resistors to the source of the direct current voltage so that the repeller voltage had the proper relationship to the motion of the contact arm.

A somewhat different arrangement used in the .past for tracking the repeller voltage of a reflex klystron oscillator with changes in the setting of the tuning plunger of the resonant cavity .has been to obtain the repeller voltage from a rectifier operating off an oscillator. Between the oscillator and rectifier, a voltage dividing network was used, the network containing a variable condenser with such a law of capacitance versus rotation that by coupling the condenser drive directly to the drive for the plunger, the alternating current input to, and hence the direct current voltage from, the rectifier yielded the desired reciprocal relationship between motion of the plunger and the value of the repeller electrode voltage.

All of the above described prior art arrangements for tracking the repeller voltage with motion of the tuning plunger of the resonant cavity have disadvantages, the principal one being an item of cost since it is quite expensive to manufacture non-linear potentiometers and non-linear mechanical -transmission movements.

It is accordingly a general object of this invention to provide an improved single-control device for tuning a reflex klystron oscillator which is simple in design, accurate in operation and which may be manufactured at low cost. Another object is to provide a tunable reflex klystron oscillator including a linearly variable series connected rheostat to control current through a fixed resistor, the contact arm of the rheostat being coupled directly with the drive for changing the setting of the plunger of the resonant cavity, and the repeller electrode being connected to one end of the fixed resistor. Another object is to pro'- vide a single-control tuning device for a reflex klystron which may be adjusted to conform with differences in electrical characteristics which are often found in klystron oscillators even though the latter are ostensibly of like construction.

In the drawings, Fig. 1 is a longitudinal central section of a klystron tube and tunable cavity oscillator of typical construction to which the improved tuning arrangement in accordance with this invention has been applied; Figs. 2, 3 and 3a are schematic views of modified forms of the invention; Fig. 4 is a group of curves showing the relationship between various operating components of the tuning device; and Fig. 5 is a family of curves illustrating the relationship between oscillation frequency of the klystron and repeller electrode voltage for various modes of operation.

Referring now to Fig. 1, the reflex klystron there shown includes an evacuated vessel I0 of glass containing an electron gun that comprises a cathode I I which serves as a source of electrons when heated by filament I2, and an accelerator grid I3. The electron beam produced by the gun passes between a pair of axially spaced resonator grids I4 connected to a resonant cavity in the form of a concentric line I5. As already explained, the modulation in velocity of the electron beam is due to the bunching effect on the electrons as they pass through the radio frequency field existing between the resonator grids I4. Upon passing through the grids, the velocity modulated electron beam enters a drift space I6 free of radio frequency field and is there reversed in direction by the static eld due to the potential on a concave repeller electrode I1 (which potential is negative With respect to the potential of cathode II). The initially velocity modulated beam is converted into a density modulated beam which now passes back between grids I4, the latter now serving as a catcher, and delivers energy in the form of ultrahigh frequency oscillations in the microwave range to a coupling loop I8 of a coaxial output terminal I9. In accordance with the usual practice the repeller current is Zero or of negligible amount since the repeller potential is kept negative with respect to the cathode.

The oscillation frequency, which depends upon the size of the resonant cavity, can be varied by changing the dimensions of the cavity. For this purpose, one end of the concentric line cavity I5 is terminated in a short circuit by an annular ring or plunger 2| that can be adjusted axially by means of a rod 22. As will be explained. later in more detail, rod 22 is caused to slide axially in a sleeve bearing formed in annular collar 23 fixed to two metallic and concentrically arranged cylindrical tubes 24, 25 that form the inner and outer walls of the resonant cavity.

The source of D. C. voltage for supplying the proper D. C. potentials to the various operating elements of the klystron oscillator is shown as being comprised of batteries 26, 21 arranged in series. Conductor 28 extending from the tap between the two batteries connects with cathode II, and conductor 29 tapped to battery 21 at its most positive point connects with the outer wall 25 of the resonant cavity.

It was previously explained that the repeller electrode is given a potential which is negative with respect to the cathode, and that this potential must be increased to more negative values as the operating frequency of the klystron is raised upon adjustment of the setting of plunger 2|. According to this invention, the required repeller electrode potential is derived from the potential drop through a resistor 3| that is connected in series with a rheostat 32 having a linear characteristic, i. e. a linear relationship exists between displacement of the rheostat arm 32a and the corresponding change in resistance introduced in the circuit by the rheostat winding 32h.

A direct, constant ratio drive connects the contact arm 32a of the rheostat with the rod 22 to effect the proper change in repeller electrode potential with changes in the setting of the plunger 2 I.

One suitable arrangement for the drive shown somewhat diagrammatically in Fig. 1 includes a shaft 33 on which are mounted in axially spaced relation an adjustable knob 34 having an index pointer 35 that sweeps over a scale 36 `suitably graduated to indicate the particular operating frequency of the klystron, the contact arm 32a of the rheostat, and a pinion 31 meshed with a rack gear 38. Rack 38 is secured to rod 22 and hence as knob 34 is rotated, the in-circuit resistance R of the rheostat will be varied linearly with longitudinal displacement of plunger 2|. Clockwise rotation of shaft 33 will effect an increase in the resistance R as the plunger 2| is moved to the right to decrease the operating frequency of the klystron, and conversely, counterclockwise rotation of shaft 33 will cause resistance R to decrease as plunger 2| moves to the left to thereby increase the operating frequency.

In the following analysis of the supply voltage circuit of Fig. 1 to show that the repeller electrode Voltage varies in the required manner with the frequency at which the klystron is operated, the voltages across batteries 26, 21 are designated E and E", respectively, and their combined voltage by Eo. R designates the incircuit resistance introduced by rheostat 32, Ro designates the resistance of fixed resistor 3|, and e which designates the voltage drop across resistor 3|, is the repeller voltage applied to repeller electrode I1 via lead 40.

Referring now to the voltage supply circuit shown in Fig. 1, it will be apparent that y With a direct mechanical drive` 33 between the rheostat arm 32a and plunger 2 I, and designating c and d being constants.

' Since ifi-:v (the velocity of propagation) Hence, from Equations 2: and 3 R=,+1 (4.) Substituting for Rin Equation 1, the latter now becomes where a and b are constants, comparison of Equations 6 and 5 indicates that if d=Ro 5:0, Equation 5 will satisfy the requirement giving from which it is apparent that the repeller potential e is directly proportional to the csciliation frequency of the klystron.

As to the amove hypothesis, if the repeller potential e is made as positive as, and equal to, the potential of the resonator grids I4, then the frequency f will go to zero for any mode. Thus the significance of making b=0 in Equation 6 is that one terminal of the source of repeller potential e should go back` to the` D. C. potential of the resonator structure. y

The significance of d=Ro can be appreciated from an examination of Equation 2. When R=0,

v \'=)\min. and hence d=-c'7\min. In order to reduce wmin. to zero (f=) by controlling R alone, R should be carried from a positive value downwardly through zero and then negatively to .a .value such that R=-Ro at which the current through (and hence the potential across) the actual Ro would be infinite. Thus at l.=0, using Equation 2 one would obtain .Rz-Sad, or (iz-Rn.

Relationships between the various factors which have been discussed are pictured graphically in curves shown in Fig. and no detailed explanation of these is believed to be necessary. The curve in the lower left quadrant shows wavelength which varies directly with the setting of plunger 2|, to be related linearly to the in-circuit resistance R of the rheostat 32; thev curve in the lower right quadrant is a plot showing the repeller voltage e obtained as the wavelength is varied by changing the plunger position; and the curve in the upper left quadrant is a plot of operating frequency f corresponding to wavelength variation assumed in the curve 1n the lower left quadrant.

By combining the information on frequency and repeller voltage contained in the upper left and lower right curves, respectively, the curve shown in the upper right hand quadrant may be produced. It will be found that the relationship between frequency and repeller voltage thus obtained is linear and thus fulfills the known tracking requirement.

In actual practice, it has been observed. that as to some types of klystron tubes, the required repeller potential e does not vary in an exactly linear manner with the oscillating frequency of the tube, but at the lower end of the range of operating frequencies is required to decrease less rapidly than at the upper end of the frequency range. This eiiect is graphically pictured-in the family of curves in Fig. 5 for different mode numbers. If this condition is found to exist, a modified form of the Fig. 1 circuit, .as shown in Fig. 2, may be. `used so that the derived repeller potential e will accurately track the curvature of one of the particular solid line curves shown in Fig. 5'.

Referring now tol Fig. 2, it will be seen. that the circuit is the same as-that shown in Fig. 1, but with a fixed resistor 39 connected in shunt with the rheostat 32. The resistance Riof resistor 39 modifies the potential drop acrossv resistor 3| fora given change in the setting of the rheostat arm 32a such that the repeller potential e falls off less rapidly with decreasing frequency at they lower end of the frequency range. The required resistance- R1 of resistor 39 will of course depend upon the shape of the voltage-frequency characteristic required by the particular klystron when operated in the desired mode over a specified tuning range.

The preceding discussion has been based on the assumption that the linear portions of the frequency-repeller potential curves of Fig, 5 when extrapolated towards zero frequency would meet at zero frequency and at the potential of the resonator grids i4, i. e. all curves would pass through the origin O. While this condition corresponds to rst order theory, it is not always met in practice. l For example, as shown in tlic curves of Fig. 5, the extrapolated curves (indicated by the dashed lines) may go to zero frequency at a repeller potential e', less positive than that of the resonator grids. Assuming that the operating condition is as shown by the curves in Fig. 5, proper compensation may be effected as shown in the modified circuit arrangementof Fig.' 3 by moving the connection for the positive end of resistor Si (Ro)Y to an intermediate tap -il along battery 2l. Alternatively the positive end of resistor 3l (Ro) may be connected to a tap'on a fixed resistor comprised of two elements Rz and Ra in series which is'connected across E" as shown in Fig. 3a. In this case the actual resistance of Ro must be reduced from that required for the battery-tap connection in Fig. 3 by the amount equivalent to the parallel resistance of R2 and R3, namely, by

-1Min Rz-l- R3 R2R3 RT- Ra I claim:

1. For use with a klystron tube having a resonator cavity and a cathode cooperating with resonator grid electrodes and with a repeller electrode for operation on the reflex principle, in combination; a repeller electrode voltage supply circuit comprising a source of direct current voltage, a fixed impedance, an electrically linear variable impedance and means connecting said fixed impedance in circuit with said repeller electrode such that the voltage of said repeller electrode relative to another electrode of said tube varies with the change in voltage drop across said fixed impedance as said variable impedance is adjusted; means for adjusting the resonator cavity of the tube; and a constant-ratio mechanical linkage between said variable impedance and said cavity adjusting means whereby the tube may be tuned by a single control.

2. The combination with a reflex klystron tube having a resonator cavity and a cathode cooperating with resonator grid electrodes and a repeller electrode for operation on the reflex principle; means for adjusting the resonator cavity of the tube to vary the wavelength substantially in proportion with displacement of the adjusting means; a source of direct current voltage having an intermediate voltage tap between the positive and negative voltage terminals thereof; connections from said tap and the positive terminal to the cathode and grid electrodes respectively; of means for maintaining the repeller electrode at a voltage, with respect to the grid electrodes, which varies substantially as the frequency at which said resonator cavity is tuned; said voltage-maintaining means comprising a xed and an electrically linear variable resistor connected in series across said voltage source with the variable resistor between the fixed resistor and the negative terminal of the voltage source, a connection from the junction of the resistors to the repeller electrode, and a constant-ratio mechanical linkage between said cavity adjusting means and said variable resistor.

3. For use with a reex klystron tube having a resonator cavity and a cathode cooperating with resonator grid electrodes and a repeller electrode, and resonator cavity adjusting means, the combination of a voltage supply system for the repeller electrode comprising a direct current voltage source, a fixed resistor, and an electrically linear variable resistor connected in series to the source; a constant ratio mechanical linkage between the cavity adjusting means and said variable resistor for varying the voltage drop across said fixed resistor substantially linearly with frequency as the wavelength is varied by the cavity adjusting means as a substantially linear function of the displacement thereof; and circuit means applying said voltage drop between the repeller electrode and another electrode of said tube having a fixed potential relative to that of said source.

4. In combination with a reflex klystron that comprises a cathode, a resonant cavity, a resonator grid and a repeller electrode for producing a density modulation of an electron beam emanating from the cathode; a tuning device for varying the operating frequency of the klystron, said tuning device comprising, means for adjusting the resonant cavity, a source of fixed direct current potential; a rheostat having a linear displacement-resistance characteristic, a fixed resistor, means connecting said rheostat and resistor in series to said potential source, means connecting said fixed resistor in circuit between the repeller electrode and resonator grid, and coupling means between the resonant cavity adjusting means and the displaceable member of said rheostat for decreasing the in-circuit resistance of the rheostat linearly with a decrease in operating wavelength of the klystron.

5. In combination with a reflex klystron tube that comprises a cathode, a resonant cavity, a resonator grid and a repeller electrode for producing a density modulation of an electron beam emanating from the cathode, a tuning device for varying the operating frequency of the klystron, said tuning device comprising, a plunger adjustable to alter the resonance' characteristic of the cavity, a source of fixed direct current potential, a rheostat having a linear displacement-resistance characteristic, a fixed resistor, means connecting the rheostat and resistor in series to'said potential source, means connecting said fixed resistor in circuit with said repeller electrode such that the repeller electrode voltage varies with the voltage drop across said fixed resistor, and a constant-ratio drive coupling the adjusting means for the plunger with the displaceable member of said rheostat to thereby effect a change in the applied repeller electrode voltage that increases linearly with an increase in oper'- ating frequency of the klystron.

6. A klystron tuning device as defined in claim 5 and further including a fixed resistor connected in parallel with said rheostat to compensate for departures in the required repeller potentialfrequency characteristic of a particular tube from a truly linear relationship.

PAUL O. FARNHAM.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,974 Linder Feb. 10, 1948 2,407,708 Kilgore et al Sept. 17, 1946 2,434,293 Stearns Jan. 13, 1948 2,434,294 Ginzton Jan. 13, 1948 2,496,535 Hoglund et al Feb. 7, 1950 2,515,203 Ernst July 18, 1950 

